In gyratory, vibratory and reciprocal type separators, or "classifiers," the particulate matter to be classified forms a particle bed on a supporting surface that is agitated with a gyratory, cyclical or repetitive motion. The gyratory type devices disclosed in the above U.S. Pat. Nos. 4,148,725 and 4,319,995, for example, classify particles of given size according to their relative densities.
Gyratory separators conventionally have been used to classify particles in accordance with size. A typical separator classifier is disclosed in U.S. Pat. No. 2,950,819, in which the particle mixture is placed upon a vibratory screen designed to pass particles of all sizes smaller than the screen openings. Such gyratory screening classifiers, are operated such that oversize particles ordinarily move to the periphery of the screen and are discharged. Particles which are smaller than the screen openings fall through into a collection chamber from which they may be extracted.
Gyratory classification devices, when used to classify particles according to relative density, can be effective classifiers, however, a significant problem has been their limited processing rate. Specifically, such devices have inherent limitations restricting the rate of flow of material to be classified. Attempts to increase the flow rate by speeding up the frequency or intensity of gyratory motion tends to impede the further flow of particles from the feed hopper into the working chamber where the separation occurs. On the other hand, attempts to add particles to the feed hopper at a faster rate only reduces the degree of fluidization of the particle bed and, ultimately, separation simply stops. That is, when material is introduced too quickly, the particles exhibit a significant reduction in activity, which can be observed as packing of the particle bed.
In screening operations, i.e., size classifications, problems are often encountered with very fine particulate matter, such as epoxy powdered material as fine as about 250 mesh. This problem is generally manifested as a "blinding" of the screen. When blinding occurs, the holes of the screen become blocked and no longer permit passage of the fines. Conventional gyratory screening classifiers make use of a gyratory container carrying a horizontal screen upon which particles to be classified are placed. As mentioned above, those particles which are finer than the screen openings fall through into a lower chamber for collection or removal. Coarse particles are ordinarily extracted at the periphery of the screen. These conventional devices sometimes make use of "scrolls" to increase the residence time of particles on the screen. To control the activity of particles on the screen, the components of gyratory motion can be adjusted.
Adjustment of gyratory motion is accomplished by (a) altering the size and relative angular position of the throw weights at the top and bottom of the drive motor and/or (b) by varying the speed of the motor. As a general rule, the bottom weight tends to control the amplitude of the vertical displacement of gyratory motion, whereas the top weight tends to control the degree of eccentricity of the motion when the weights are angularly displaced. In operating screening classifiers, the object is to present the particles to the open apertures of the screen as frequently as possible so that the fines can drop through the apertures before leaving the screen. In order to achieve this object, a balance must be struck between the flow rate through the machine and classification efficiency. Attempts to increase the throughput rate in conventional screening devices have generally resulted in reduced screening efficiency; conversely, attempts to improve efficiency have generally required a reduced rate of throughput.
Similarly, in density classification in which the object is to separate dense or higher mass particles from the bed, the feed rate which can be achieved without overloading the particle bed or diminishing the residence time unacceptably has been restricted. It was found that increasing the frequency of gyration often merely caused particles to back up into the feed hopper. The reason for this behavior of the particles is not completely understood, but appeared to be the result of an increase of pressure in the classification head due to the increased counter throw of the bed supporting surface.